Even simple lies like this stress me out. My method is to just abruptly say, "sorry, I've got to go" and hang up. If asked about it later, "I'd really rather not talk about it." They pretty much never ask about it later though. Zero lies, zero stress, and you don't have to keep talking on the phone.
I had a friend long ago that would say "I'd help you move, but I don't want to." That has stuck with me ever since. I don't make excuses for not wanting to do whatever. They get over it, and you get a reputation for being "direct" or "no bullshit."
Pro tip:
Disable GSM (2G), disable WCDMA (3G), disable VoWiFi, disable VoLTE, disable VoNR.
You still get access to mobile data via 4G and 5G, but you just disabled all technology which allows phone calls. It's now literally impossible.
Drawback: It applies to outgoing phone calls as well. This includes emergency phone calls!!!
A less stupid method:
Check call forwarding settings, copy the number from "when unreachable" to "when busy". If everything goes right, the caller gets "The number you have dialed is temporarily unavailable..." or a similar message when you decline a phone call. For me this breaks with VoLTE enabled, and for the caller it seems as if it just kept ringing. It does however work with GSM-only.
Drawback: This method requires MMI to access call forwarding settings. MMI is only available on 2G and 3G, so your carrier needs to have those. But they may have alternative method for setting call forwarding, or perhaps there's some way MMI (and USSD codes) may work over IMS (the carrier would need to support that). I don't know, I am dumb.
Well... I am using the first method. I usually have a secondary cellphone with me, as a backup, so I could use that for emergency calls ¯\_(ツ)_/¯
LPT or masking tip? Masking makes things more bearable but I find it often prevents being able to make a genuine connection. I can mask fairly well for a couple hours but by then I'm exhausted. Trying to mask with people I see regularly inevitably ends up in ruined relationships due to internalized guilt from lying and them recognizing me as a liar. It saves time and stress to just be honest, even if that means coming off as acerbic or just eccentric.
More like a joke, but I bet the tip would work with people that historically do not respect boundaries but you still have to maintain a working relationship with. But yeah, I've been on a de-masking kick lately, and it's so much nicer. Less stress and I start building relationships with people I can be myself around.
I just dont answer calls from anyone other than my girlfriend or closest friends and they know me well enough that I can just say that I can't talk rn and they'll understand