I have a similar problem, but with the normal (i.e. non-mobile) phone. I currently use Firefox 14.0.1 on Linux Mint Maya (I was a Ubuntu fan until the wretched Unity came along with 12.04), but I have the same problem if I revert to earlier Ubuntus, and with Kubuntu and Fedora.
My bandwidth is limited to about130 KB/sec since we live in the French countryside at the extreme end of an old telephone line a little over 3 miles long. Recently when I make a download which is at or near the limit of the bandwidth, the telephone is cut off. If a call is in progress, it stops; if I try to make a call, I get only the engaged tone. As soon as the download ends, the phone returns to normal. If I download at lesser bandwidth – e.g. stream music from a radio station at some 50 KB/sec - there is no problem with the telephone at all. Changing from WiFi to ethernet connection makes no difference.
However, the really bizarre bit is that if I make the same 130 KB/s download from the same (dual boot) computer using Window$ XP (shudder!), there is absolutely no interruption to the telephone. Also Win 7 on my wife's portable causes no problem with the phone.
My ISP claims that the interruption is caused by the length of my phone line and that it is not their fault. But if this is the case, I don't see (and they refuse to explain) why Window$ does not produce the same problem. And surely the filter (which, incidentally, I have changed in case it was the problem) is there to isolate phone from internet. Their advice, shorn of all the French niceties, is "use Window$". I won't print my reply to that in case it causes offence, but has anyone out there better advice? Is the Linux-based internet connection in some way more aggressive than Window$, so grabbing more bandwidth to itself? If so, can I turn it down a bit??