Originally Posted by XDASenior
reflash your firmware or just flash (.zip) a rom
That wasn't the question what was asked. The OP is after an answerphone app, preferably not their network answerphone.
Anyway, OP. I don't think it is possible to create such an app, you would need to take over the phone state and then answer the call on incoming and then take and store a message. It may be possible however, I don't think it would be though.
Sent From My C1905
My Phone: Sony Xperia M C1905 | ROM: Modified Stock | Base Firmware/Build Number: 15.4.A.0.23 | Android Version: 4.3 | Kernel: Stock (Permanently Locked Bootloader) | Network: GiffGaff
My Tablet: Google Nexus 7 (2012) | ROM: CM11 | Base Firmware/Build Number: 11-20140527-NIGHTLY-grouper | Android Version: 4.4.4 | Kernel: CM11 | Network: N/A