I think the only thing that a sat phone and plb have in common are that they both utilize satellites. A sat phone is a two way communication device carrying voice data where a plb is a one way communication device carrying location data and emitting a beacon signal.

Sat phones can be use to contact others in non-emergency scenarios whereas plb use is limited to serious injury (e.g. loss of limb or eyesight) or potential loss of life (e.g. a last resort option).

A number to the local SAR agency for the area you will be in can be programmed into the emergency number memory and can be dialed even if the phone is locked (user configurable). Typically I will have some numbers for local help prepared and in my pack prior to my activity begins (e.g. outfitter, local and state police (or equivalent). I do not lock my phone or the sim card. I feel there is no reason to do so and doing so will only inhibit someone else in your group from calling for help.

Typically I will pack both a sat phone and plb for my Canadian sled trips which are a wilderness setting being there are times when the closest human being is eighty miles away. With both a sat phone and plb I am able to check in with family and also have a last resort option to summon help.

Sat phone air time is not free and there are two ways to pay for it. Prepaid and monthly. For prepaid there are several flavors of minutes and when they expire. These can give a low per minutes cost since payment is up front but will only do so if the minutes are completely used before they expire. It would be a bummer to have your sim expire during a scenario of need. I typically go with monthly turning the phone on before my fun season and off afterwards. Depending on the iridium air time reseller monthly service is around $30 to $40 a month plus air time which can be between just under $1.00 (USD) to $2.00 USD) per minute