Don’t have any recommendations here, just thought I’d mention that for the stated purpose—preventing the tracking of the cellphone location (i.e. where you go)—running your own VoIP to PSTN/ISDN/GSM bridge at home would be sufficient.
Completely anonymous hosted PSTN bridge would be very open to abuse, so I imagine not many of them are/will remain accessible via Tor. But for completely secure and/or anonymous communication you don’t need or want PSTN, all you need is (encrypted) VoIP–VoIP that you control, and can optionally expose via Tor (as a hidden service).
Bonus points: GSM bridge at home raises less flags. You could even “take it for a walk” from time to time to make it appear even less suspicious.
