There was a GSM-to-VoIP node based on a femtocell but the company making it apparently abandoned the idea before shipping very many, if any at all. I suspect the telcos had a small freakout.

In theory one can be transmitting on licensed spectrum inside the home without permits, but I suppose the telcos never envisioned a "3rd party femtocell" that wasn't connected to their services. So I imagine they were faced with drowning under deluge of telco lawyers.

Still, that was a brilliant idea. I think the product was eventually pivoted to shipboard applications. But, with unlimited voice calls on many plans, a roam-to-voip home base station holds less appeal.