If you want to succeed in such a trade, you really have to dedicate yourself to all that is involved: write texts. produce courses, read, converse/network, and put yourself available to your every company for as long as possible. It is possible to reach the lowest levels of pertinence to human kind, since I call pertinence possession of human rights, even through one company only, but this company would have to be a big one, so say TIS, and your language would have to be really needed. Most professional interpreters would serve at least four companies to reach the lowest levels of pertinence to human kind. You could then have privacy (not share a place with anyone), lowest levels of property (a telephone set, furniture, basic clothes, etc.), and freedom (a few choices, very limited ones, but still a few). You would be making about 40 K per year. That would mean that if you kept your budget really low in terms of costs (telephone, gas, electricity, Internet, food, rental, etc.), you could have the money to pay the initial deposit for your own property, in case you conformed with the super basic again, in two years' time. That would mean nothing beyond the super basic, of course (no restaurants, no parties, no pubs, no car or motorcycle, no nothing).
For that, you would be logged from 7:30 AM to 11:00 PM every day except for weekends, when you could log in for just a few hours, but you would probably still have to log. Onsite jobs usually pay more and give you more prestige, but they also give you more work (you have to make use of public transport, invest in better clothes, spend a few hours going and coming without getting paid, invoicing people, etc.). You can then get compliments in writing, what is very hard to get with telephonic interpreting. The advantages to the side of the onsite interpreting do not resume to compliments and higher pay, but they also include not depending on electronic equipment and on things you cannot do anything to change, so say quality of the speakers.
Onsite jobs give you more freedom, a bit of physical exercise (moving), a bit of social interaction, so that they are better for your health in general (physical and mental), more compliments in writing (things that you can keep and show), and more money. You will perhaps experience loss of money instead of gain or almost that if you go to your assignments by car instead of by public transport or feet if you do onsite jobs. Cabs are still better than your own car (insurance, fuel, chance of having an accident, parking, etc.).
In compensation, during the time you are doing those jobs, you could be doing three calls (putting it all together: going and coming, serving, invoicing, etc.). You definitely risk losing your voice and ears more if you do telephonic interpreting. You risk acquiring cancer (mobile telephone, radiation) and other diseases, so lesion of the repetitive effort (holding the telephone set, typing in the computer, etc.), more if you do telephonic interpreting. You spend less money (you don't have to invest in clothes at all), and it is all more convenient (you do not have to leave home, etc.) if you do telephonic interpreting. If you do telephonic interpreting, it is very unlikely that you will ever get notified of any compliments received. Complaints, however, will always get to you very quickly and in a very disgusting way. When you do onsite, your compliments may also get lost in the system, so you must worry about making photocopies of those before giving them to the boss together with your invoice or material. I worked for a company where they told me they dumped them all without ever computing, believe it or not (and I only got to know that by the zillionth compliment received!). I tested the system, and I can guarantee that plenty of compliments received in telephonic interpreting will never ever be acknowledged by the boss or computed. You are never going to know about them. With the onsite jobs, however, they cannot do that to us: We see them, we see the clients face to face, we hear them, etc.
If you do telephone interpreting, you may receive a call when a friend comes to speak to you on the street. You will then have to interrupt your conversation or not converse at all, so that you may look like a wacko. If you do onsite, you will not have this sort of problem, since it is like having a normal job when it comes to this sort of situation. You will also be highly distressed with the quality of your equipment, the surrounding sounds, and all else. If the fire alarm rings, you will be so distressed that you may acquire a gray string of hair on the spot. Even though that could be seen as normal, you could be serving the minister or immigration when that happens, and that could look and sound really bad. You will cause a lot of inconvenience if you share a place with other people or even if you live in a place that is populated, and really cheap (with little protection in terms of sound isolation), so that you should prefer living on your own. You may get a call in the middle of the night because immigration cannot find someone else or because there is an emergency of some sort. If you don't get after hours or weekend calls, you will never be able to get the 40K per year, so that you would have to be praying to get those every week, basically. Sometimes you will be waiting for hours until the recording finishes, so say you are serving Telstra, Optus or immigration. If you happen to be on the street and, say, your private student pops up, as it has happened to me, you will feel a lot distressed, because, first of all, you look like an idiot: Nobody is speaking to you, you have to keep the watch, and the other person obviously cannot understand why you are pretending to be busy or something. You know, some people hold the telephone set to simply pretend to be busy and things like that.
In terms of bosses, the chance for abuse is also much higher for the telephonic interpreting because, with the onsite, there are always more people around when you visit the company to drop invoices or when you are called to speak to one of them.
Telephonic Interpreting involves interacting with people that you are unlikely to meet in life, so that it will always be cold and distant, but, even so, you are obliged to be polite at all times. Onsite Interpreting makes you see the face of everyone, so that you may acquire a little family in that group you have there, at work. If nothing else, their Christmas parties may interest you and your partner.
There is a lot of repetition involved in telephonic interpreting, but the onsite jobs are likely to always give you some novelty. With time, you will know the scripts of some companies by heart. That may be good, since if the quality of the equipment is bad, you can always complete from memory, but it may also be irritating, and change you into the equivalent to a robot.
If you work too much with telephonic interpreting, you may become a bit of a bush person because you start getting used not to go out and to, for instance, frequently go to the toilet, and receive calls even so. You may also relax because there are no calls for a while, and decide to then have a shower. By the moment you are under it, full of soap, all wet, the telephone will ring. You will then get used to grab the towel, interrupt all, and get out to serve the clients. That will make you believe that you relate to others, and that you have a social life, since a good share of our social lives, if we are commoners, is spent on the telephone. That is a problem: Those are your clients, not your friends. You are not having any social interaction or life. You are simply working as a machine would. Given that you will really have to be available for this amount of time that I am telling you to make some basic money, you are a slave of the trade, and you are then realistically losing quite a lot in what regards life and living.
All other professions allow you to have regular hours, paid leave, constant parties, get togethers, course allowances, opportunities, etc. You will also have regular hours for the meals, normal social life, etc. This is an extremely sacrificed profession in all senses. The amount of investment you would have to put into becoming good at this trade is equivalent to at least 20 years for the languages in terms of learning (10 for each), 12 years of training in Logic (Mathematics, Philosophy, etc.), and one year of specialized training (PG Dip or whatever). Simple calculations may prove this to be a bad choice, is it not? The activity is however very noble in all its intentions.
There is much more disrespect for telephonic interpreters, and disrespect coming from everywhere (operators, managers, primary and secondary clients, etc.), than there is for onsite ones, as another point.
If you read the book Translation and Interpreting, vol. 1, you may get a good idea about the trade and the difficulties involved. If you do our course on ethics, more will come to you in terms of understanding all, but nothing is better than practice, is it not?