Analog phone calls over mobile

I bought a cool old analog phone off ebay and thought it would be sad if it is only used as decoration. I hooked it up to our landline and turns out it works just fine, but it is 2025 and nobody knows or calls our landline number. So I did some research and found this blog post describing how to route your phone calls through your mobile using an ATA (Analog to VOIP/SIP converter) and a Raspberry Pi running asterisk connecting via bluetooth to your mobile phone.
After desperately trying to use an old FritzBox as an ATA talking to my Raspberry Pi I wanted to ditch the ATA and I am now pursuing these (totally easier, no not really) solutions:

O2 6431 router

• running OpenWRT
• supports builtin FXS (analog telephone) ports using chan_mobile asterisk module
• flash size problem -> overlayfs on usb stick
• bluetooth via usb adapter
• performance issues running asterisk & bluetooth together, not working yet

ISDN card

• HFC-4S PCI ISDN adapter supporting asterisk from an ebay auction for 3 EUR
• DETEWE TA 33 ISDN - analog adapter for 2 EUR
• now I am testing this setup using my dad's old Siemens-Fujitsu T-Bird PC from the early 2000s, because I do not own any device supporting PCI (without the e)