My idea is clear from the name. I doubt I'm the first who has thought about it, but in fact this is not important. Today 16.jan.2011 I was thinking about GPSDO or some stable time base. The main goal is to be cost effective solution for everyday use in my shack. GPSDO or rubidium oscillators are cheap enough these days but each GSM/3G network uses stable time base and each active subscriber is locked to it, so your old mobile phone can be usefull, but only if you have active SIM/USIM at a reasonable price. The VCTCXO can be at 13MHz, 26MHz, 38.4MHz etc. If this frequency is not suitable for your needs a simple PLL and 10MHz VCTCXO can be locked to mobile's VCTCXO. I expect stability better than 0.01 PPM, not good as GPSDO. The stability also may be worse in some mobile phones. Since i do not have any other stable time base and test equipment, even if I realize this idea, I will not be able to tell how stable is the time base of my local mobile operator and phone, but if you test this someday i will be happy to receive comments from you. 73!